During orthopedic surgery, a soft tissue wound must be made to gain access to the bones to be repaired and a bone wound is present from the original defect and from the process of treating the bone. Both types of wounds cause tissue responses that are well characterized but not well understood. Park and Lakes describe these processes in their Biomaterials book, Vol. 3, as follows, with the figure numbers being revised for consistency with the present patent application:
10-1. Normal Wound-Healing Processes
10.1.1. Inflammation
Whenever tissues are injured or destroyed, the adjacent cells respond to repair them. An immediate response to any injury is the inflammatory reaction. Soon after injury, constriction of capillaries occurs (stopping blood leakage); then dilation. Simultaneously there is greatly increased activity in the endothelial cells lining the capillaries. The capillaries become covered by adjacent leukocytes, erythrocytes, and platelets (formed elements of blood). Concurrently with vasodilation, leakage of plasma from capillaries occurs. The leaked fluid combined with the migrating leukocytes and dead tissue will constitute exudate. When enough cells are accumulated by lysis, the exudate becomes pus. It is important to know that pus can sometimes occur in nonbacterial (aseptic) inflammation.
At the time of damage to the capillaries, the local lymphatics are also damaged since they are more fragile than the capillaries. However, the leakage of fluids from capillaries will provide fibrinogen and other formed elements of the blood, which will quickly plug the damaged lymphatics, thus localizing the inflammatory reaction.
All of the reactions mentioned above—vasodilation of capillaries, leakage of fluid into the extravascular space, and plugging of lymphatics—will provide the classic inflammatory signs: redness, swelling, and heat, which can lead to local pain. • • •
10.1.2 Cellular Response to Repair
Soon after injury the mesenchymal cells evolve into migratory fibroblasts that move into the injured site while the necrotic debris, blood clots, etc. are removed by the granulocytes and macrophages. The inflammatory exudate contains fibrinogen, which is converted into fibrin by enzymes released through blood and tissue cells. The fibrin scaffolds the injured site. The migrating fibroblasts use the fibrin scaffold as a framework onto which the collagen is deposited. New capillaries are formed following the migration of fibroblasts, and the fibrin scaffold is removed by the fibrinolytic enzymes activated by the endothelial cells. The endothelial cells, together with the fibroblasts, liberate collagenase, which limits the collagen content of the wound.
After 2 to 4 weeks of fibroblastic activities, the wound undergoes remodeling, during which the glycoprotein and polysaccharide content of the scar tissue decreases and the number of synthesizing fibroblasts also decreases. A new balance of collagen synthesis and dissolution is reached, and the maturation phase of the wound begins. The time required for the wound-healing process varies for various tissues, although the basic steps described here can be applied in all connective tissue wound-healing processes.
The healing of soft tissues—especially the healing of skin wounds—has been studied intensively since this is germane to all surgery. The degree of healing can be determined by histochemical or physical parameters. A combined method will give a better understanding of the wound healing process. FIG. 1A shows a schematic diagram of sequential events of the cellular response of soft tissues after injury. • • •
The healing of bone fracture is regenerative rather than simple repair. The only other tissue that truly regenerates in humans is liver. However, the extent of regeneration is limited in humans. The cellular events following fracture of bone are illustrated in FIG. 1B.
When a bone is fractured, many blood vessels (including those in the adjacent soft tissues) hemorrhage and form a blood clot around the fracture site. Shortly after fracture the fibroblasts in outer layer of the periosteum and the osteogenic cells in the inner layer of the periosteum migrate and proliferate toward the injured site. These cells lay down a fibrous collagen matrix called a callus. Osteoblasts evolved from the osteogenic cells near the bone surfaces start to calcify the callus into trabeculae, which are the structural elements of spongy bone. The osteogenic cells migrating further away from an established blood supply become chondroblasts, which lay down cartilage. Thus, after 2 to 4 weeks the periosteal callus is made of three parts, as shown in FIG. 1C.
Simultaneous with external callus formation, a similar repair process occurs in the marrow cavity. Since there is an abundant supply of blood, the cavity turns into callus rather quickly and becomes fibrous or spongy bone.
New trabeculae develop in the fracture site by appositional growth, and the spongy bone turns into compact bone. This maturation process begins after about 4 weeks.
From the above citation, note that in this widely accepted model by Parks and Lakes, the callus shown in FIG. 1C is massive compared to the thickness of the original, now dead, cortical bone shown at the bottom of the figure. This massive callus is necessary because the fibrin and chondroital structure is much weaker than the dense cortical bone.
The formation of new bone follows a well-known sequence. First, osteoclasts remove necrotic tissue. Then, osteoblasts lay down a collagenous matrix of a specific shape. After the soft tissue's basic form is complete, mineralization of the bone occurs to develop the final properties. Prior formation of the soft tissue is inherent in bone formation.
The dominant mineral in bone is an impure hydroxyapatite, commonly denoted Ca10(PO4)6(OH)2, with a Ca:P ratio of about 1.62:1, instead of 1.67 as indicated in the chemical formula. Experimentally, calcium phosphates with a Ca:P ratio between 1:1 and 2:1 are not walled off by a tissue foreign body response. For this reason, scientists have studied calcium phosphate for bone repair. A recent review by S. V. Dorozhkin, 2009, cited 382 scientific papers in which various calcium phosphate formulations were studied for use in bone repair. He concluded that formulations based upon calcium phosphates do not have sufficient strength for bone graft applications.
Advancements have been made in the use of calcium phosphates in situations where non-load-bearing bone regeneration is enhanced. However, calcium phosphates themselves have not been successfully used in load-bearing graft applications. This is in agreement with the 1974 U.S. Pat. No. 3,787,900 by McGee that showed that a second inert phase is needed to obtain enduring strength. The content of U.S. Pat. No. 3,787,900, McGee, issued Jan. 29, 1974, is incorporated by reference into this application.
Artificial Bone Grafts:
McGee is the only inventor that has succeeded in making artificial bone grafts that are functional under load. He used the geometry of what was labeled the “osteoceramic” bone graft to allow the bone to develop strength for torsional, tensile, compressive, and bending loads, and to ensure a blood supply to augment bone attachment to the implant. Research has been conducted to attempt to find other load-bearing artificial bone grafts. These are often identified as Bone Graft Substitutes. An excellent review of the state of the art of bone graft substitutes is given in Bone Graft Substitutes, CT Laurencin, Ed., (2003) written by a committee from ASTM International in conjunction with THE AMERICAN ACADEMY OF ORTHOPEDIC SURGEONS. Thirty-five experts provide the clinical, scientific, and practical aspects of bone graft substitutes in 17 chapters. The preface has an explanation that the state of the art is for Bone Void Fillers, not for bone graft substitutes. This research can be subdivided into two categories: Tissue Engineering and Bone Void Fillers because they have different theoretical bases. The contents of this book constitute a basis for the state of the prior art.
Tissue Engineering:
Tissue Engineering is a term applied to attempts to use artificial materials or systems to invoke the tissue to repair itself in situations where it normally will not do so. The initial effort was to use bulk calcium phosphates, but that failed because the calcium phosphate compounds are too weak. It is quite clear that dense calcium phosphates are unsuitable for load-bearing replacement of functional bone. Subsequently, tissue engineering evolved to support a theoretical approach, usually requiring three components that may have different names assigned to them by different authorities. The three are a source of osteogenic cells, a scaffold on which the cells can be grown, and a source of stimulants to control and enhance the proliferation and differentiation of osteogenic cells. The usual osteogenic cells are osteoblasts that are harvested from the patient and grown in cultures in vitro. The components are cultured together in vitro until bone is well developed on the scaffold. Then the implant is placed in a bony defect with the expectation that it will be replaced with new bone by the adjacent tissue, becoming strong enough to be functional. In load-bearing bone this expectation has not been fulfilled. A second expectation is that the entire assembly will be absorbed by the adjacent bone and replacement bone will appear at the rate the implant is absorbed. This, too, has not been fulfilled.
Although the bone generation process is very complex, only osteoblast cells have been studied extensively to try to induce bone to form. This approach neglects the function of other cells, enzymes, and mechanisms that participate. This is a serious flaw in the theory. Glowing reports of success have been claimed in scientific reports. The best results have been where the patient's own osteoblast cells have been cultured in vitro on a scaffold of porous calcium phosphate together with growth factors or other bone stimulants. New bone has formed on the scaffolds, but it has not been strong load-bearing bone. Ten times the concentration of cells is required than is necessary for natural bone generation processes. Porous calcium phosphate structures with large surface areas are inherently weak. The new bone is too disorganized to fit the structure and is structurally weak. It is necessary for that structure and the new bone to be replaced by cortical bone before it can function under load. Tissue engineering has not yet achieved functional bone under load-bearing conditions.
Bone Void Fillers:
Bone void fillers are in commercial use in current orthopedic surgery. They are widely used as osteoinductive aids in many applications, especially in fusion of vertebrae, oral surgery, and periodontics. The osteoinductive nature of Demineralized Bone Matrix (DBM) was discovered in 1965 by M R Urist who found it induced ectopic bone formation in rat muscle. He postulated that specific proteins within the DBM were responsible for the activity and classified them as Bone Morphogenic Proteins (BMP). After animal experiments, the first use in humans occurred in 1975 when B. M. Libin, H. L. Ward and L. Fishman used lyophilized allograft DBM particles to treat periodontal defects. BMP was found most suitable for applications where it was contained in some way, because uncontained particles would migrate. In 1983, Urist extracted BMP from the insoluble cross-linked collagen in DBM. In 1988, J M Wozney, et al., identified the generic sequences of BMP and identified important components, BMP-1 to BMP-15. BMP-1 is not a growth factor. BMP-8 is important to muscle formation. Mixtures of MBP-2 and BMP-7, also known as osteogenic protein 1 (OP-1), have been exploited in commercial products, often combined with DBM and/or a collagen carrier.
The use of these BMP in humans is restricted by the FDA to void filling applications where they supplement the use of marrow, blood and cancellous bones harvested from the patient to enhance the kinetics of bone regeneration and where fixation of the bones in position is accomplished by other means. They are no better than the autologous sources. Many variations in composition and carriers are available. They often include Transformation Growth Factors (TGFs), which are very complex. More than 100 TGFs have been identified. TGFs are believed to attach to active sites on the surface of cells and their functions can be reversed by the presence of other growth factors. Many of the necessary features of tissue formation are still unknown. Scientific advances in this phase of tissue engineering have been achieved in recent years but much more research is needed. When used without bone grafts to support the void area, in the words of a spinal surgeon, “They just squish out.”